The present invention relates to a method for correcting variations in intensity in fiber optic chemical sensors based on wavelength discrimination of indicator dye combinations.
Various measuring devices are known for measuring a physical parameter by measuring the intensity of a light beam. Such measuring devices are used for measuring parameters such as pressure, temperature, and flow rate. These fiber optic measuring devices usually include a fiber optical transmission line or cable, a light source illuminating the first end of the transmission line, a sensor or transducer arranged on the second end of the transmission line for measuring the physical parameter by means of the transmitted light and for returning a corresponding light intensity into the second end of the line, a branching element or coupler on the first end side, a photodetector for measuring the returned light, and a signal processing device connected to the photodetector for issuing an output signal in accordance with the parameter.
The sensor on the measurement side of such a measuring device may contain a reflector which is movable in dependence on the physical parameter either perpendicularly or parallel to the end face of the fiber optical transmission line.
In such measuring devices, th fiber optical transmission line is exposed to the influence of the environment, for example, to temperature changes, mechanical stress, irradiation, vibrations, etc. These effects or influences caused by the effects may create light losses in the fiber optic transmission line. These and other effects, for example, bending of the fibers, may introduce errors in the measurements and therefore may result in erroneous readings. For this reason, means are required to compensate for such errors.
Fiber optic chemical sensors involve the absorption or emission of light by an analyte sensitive reagent at the end of a fiber optic lead. The intensity of light transmitted through or emitted by the reagent is a function of the analyte concentration in a certain wavelength region of the light. Typical examples of this are an absorptiometric color indicator for pH measurement, such as phenol red, where green light is absorbed by the dye as a function of increasing pH, so that the transmittance of green light through the dye indicator decreases with pH. Another such example is a fluorescent indicator dye, such as that used for oxygen measurement, where the dye is excited to fluorescence by blue or ultraviolet light and the fluorescent green light which is emitted varies in intensity with oxygen pressure. Other types of fiber optic sensors are similar, with the intensity of the light returning along the fiber to the measuring instrument from the sensing end aarying with the analyte concentration. It is desirable to have some method of correcting for variations in the intensity of light in the fiber optic system which are not analyte dependent, particularly those variations which result from fiber bending, and for variations in the illumination source.
With a fiber optic sensor which consists of two optical fibers joined at or before the sensor end, with one fiber collecting light from the illumination system and the other fiber returning light to the measurement system, such compensation has been relatively simple, as shown in FIG. 1. In the case of an absorptiometric indicator, the illuminating light can consist of two wavelength regions which pass through the dye indicator and back along the other fiber to the measurement system. Both wavelength regions have a common optical path. Light of one wavelength region is absorbed by the indicator as a function of analyte concentration, and the other wavelength region is not absorbed by the indicator. As a result, the ratio of the intensities of these two wavelength regions provides a measure of analyte concentration, and other optical variations (common path variations) cancel out. In the case of a fluorescent indicator, the same system has been used, with the excitation wavelength region of light returning to the measurement system along with the analyte sensitive fluorescent light in a different wavelength region, and the ratio of the intensities of the two wavelength regions compensates for common path variations. Existing fiber optic chemical sensors are based on this (cf. Peterson et al., Analytical Chemistry 52, 864 (1980); and ibid 56, 62 (1984); and U.S. Pat. 4,200,110.
Single fiber chemical sensors can also be made. Here, a single fiber with analyte sensitive reagent at one end is connected at its other end to the measuring instrument. An optical system in the instrument injects light into the end of the single fiber and observes light coming out of the same fiber for measurement. There are fuur general methods of combining and separating the entrance and exit beams in a single fiber chemical sensor.
The first is to use a bifurcated fiber or coupler which joins two fibers into one. This is equivalent to the dual fiber system.
The second is to use a partially reflecting mirror arranged so that the illumination light is reflected into the end of the single fiber, some being lost by passage through the mirror, as shown in FIG. 2. Light returning from the fiber passes through the mirror to a measuring system, with some light being lost by reflection. This achieves the same result as a dual fiber system but is not attractive because of the large light loss at the partially reflecting mirror.
The third method is to use a spatial filter, which makes use of the fact that light can be launched into a fiber in a collimated beam of diameter similar to the fiber diameter, and light exits from the fiber in a conical path so that it can be collected by a lens or reflector of large diameter, losing only the light which exits along the small diameter entrance path, as shown in FIG. 3. With this arrangement, the same compensating methods described for the dual fiber sensor can be used, although stray light problems make an alternative system attractive.
Ruell et al., in U.S. Pat. No. 4,356,396, disclose a fiber optical measuring device which compensates for losses in optical joints by passing a first wavelength through a mirror in front of a sensor and a second wavelength through the sensor and to a mirror behind the sensor. Both wavelengths are then reflected back to a detector and a ratio of intensities of the signals is obtained.
Another fiber optic system for measuring chemical and/or physical quantities is shown in Brogardh et al., U.S. Pat. No. 4,446,366. The device used has a monitoring transducer with a response spectrum which within the emission spectrum of the incident light source gives rise to an emitted light spectrum of the incident light on he quantity to be measured, which emitted light spectrum emanates from the measuring transducer, and at least one wavelength interval of the emission spectrum of the incident light source has a dependence on the quantity to be measured. Tne quantity to be measured is not identical to the corresponding dependence in at least one other non-identical wavelength interval of the emission spectrum of the incident light source. The optical fiber means has filtering spectra which divide the emitted spectrum, emanating from the transducer into at least three non-identical wavelength intervals, which can overlap, and a photo detector which is arranged to measure the emitted light after filtering and generate detector signals in the respective wavelength interval. The measuring transducer consists of a material with an optical absorption edge which, within one wavelength interval, coincides with the emission spectrum, and the dependence of the response spectrum of the measuring transducer on the quantity to be measured involves wavelength displacement and/or deformation of the absorption edge. Alternatively, the measuring transducer may consist of at least one filter of interference type haying transmission or reflection spectra which vary with the quantity to be measured within the emission spectrum. None of the prior art shows the use of light emitted at two wavelengths, one which results in fluorescence which is analyte sensitive and the other which results in fluorescence which is analyte non-sensitive.